Silicon nitride has a wide variety of uses based on its physical and chemical properties. These uses, to name a few, include thermocouple protection tubes, crucibles for foundry use, substrates for electronic applications and structural components for gas turbine engines.
Silicon nitride can be produced by a number of different processing techniques with each technique yielding a different final density. Each technique also has a definite restriction on the final shape which may be produced. Simple shapes of better than 98% of theoretical density can be made by hot pressing silicon nitride powder to form the final article. Complex shapes, however, generally cannot be manufactured by this processing technique.
As an alternate to the hot pressing technique, silicon nitride articles of complex shape, having densities of 70 to 75% of theoretical density are produced by an injection molding technique. In this technique, silicon metal particles and a thermoplastic are formed into a mixture. This mixture is injection molded to form the shape of the article. Subsequent operations include the heating of the article to burn out the thermoplastic and a nitriding operation to produce the final silicon nitride article.
It is generally impossible to fabricate a complete rotor for a gas turbine engine of hot pressed, silicon nitride material. The impossibility of manufacturing such a complete rotor by a hot pressing technique comes about because of the complex shape of the rotor blades. The complex shape of such blades can be formed easily by an injection molding technique. It is impossible, however, to form a complete rotor by an injection molding technique as the hub portion of the rotor formed by such a technique cannot withstand the mechanical and the thermal stresses imposed on that portion of the rotor during use in an engine.
This invention teaches a method wherein the best characteristics of hot pressed and injection molded silicon nitride materials are brought together to form a complex article of manufacture such as a rotor for a gas turbine engine. The structure is bonded together by strong and uniform bonds at the junction between the different materials.